Sql Union

How to use Sql Union? Smart way to combine sql results.


This sql command is a simple concept. It combines two or more sql results. Each subsequent sql result is appended to the previous result.

Things to remember:
- each sql must have the same number of columns.
- data types must be compatible
- the column's order must be the same.

The clause ALL that is added to Sql Union is to avoid elimination of duplicate entries. That is, use UNION only if you want the program to select distinct values during the merge, and use UNION ALL if you want to select all data.

The Syntax
SELECT column_list
FROM   table_a
UNION
SELECT column_list
FROM   table_b 

AND

SELECT column_list
FROM   table_a
UNION ALL
SELECT column_list
FROM   table_b 

Example

Let's use the following simple tables to clarify.

Table Tbl_A
IDAB
1One30
3Two50
5Three70
7Four110
9Five130
11Six170
13Seven190
 
Table Tbl_B
IDCD
7Four110
8Egg85
9Fish95
 

Sql Command

SELECT ID,A,B
FROM Tbl_A
UNION
SELECT ID,C,D
FROM Tbl_B

Results

IDAB
1One30
3Two50
5Three70
7Four110
8Egg85
9Fish95
9Five130
11Six170
13Seven190

Please note ID=7 appears in Tbl_A and Tbl_B. Since Sql Union only selects distinct rows, it elimates duplicate and shows only one. Notice that the result is sorted.

Compare with this one.

Sql Command

SELECT ID,A,B
FROM Tbl_A
UNION  ALL
SELECT ID,C,D
FROM Tbl_B

Results

IDAB
1One30
3Two50
5Three70
7Four110
9Five130
11Six170
13Seven190
7Four110
8Egg85
9Fish95

One more thing, notice the row order has not changed either. This Sql Union with ALL keywrod runs faster

Quick link. From here, you can learn the following:

Data Creation

Sql Create Database
Use this to create a database.

Sql Create Table
This Sql Create Table will be used very often throughout the tutorial. It is used to create a table, in our case, mainly to create sample tables.

Sql Alter Table
Used less often, but it is very handy to know this command. With this you can basically edit a table structure.

Sql Drop Table
Used to delete a table, not just the content. This command is also used very often during the tutorial. So, you need to master this too.

Data Manipulation

Sql Insert
To add rows to a table, you use Sql Insert command. There are two methods, one is with the VALUES keyword and second is with a SELECT statement.

Sql Update
To change existing rows, you use Sql Update command. You can use it to change values in a single row, groups of rows or all rows. You can also update a table based on values in other tables.

Sql Delete
To remove rows, you use Sql Delete command. You can delete one rows, group of rows or all rows. You can also delete rows based on values in other tables.

Data Retrieval

Sql Select
The core of Sql, selecting data from table.

Sql Distinct
Eliminating duplicate values in the output.

Sql Where
Conditionally selecting rows from table.

Sql Like
Pattern searching for conditions in Where or Having clause

Sql Between
Added to conditions to show results between two values.

Sql In
It enables us to specify multiple values, either as a list or a subquery.

Sql And Or
Connecting two or more conditions in the Where or Having clause.

Sql Count
Counting number of rows.

Sql Sum
Getting the sum of a field

Sql Max
This function will return the highest value.

Sql Min
This function will return the lowest value.

Sql Avg
This function will return the average value.

Sql Order By
Sorting the output

Sql Group By
Grouping rows into sets.

Sql Having
To filter the result of aggregation

Advanced Sql Tutorial

Sql Inner Join
Use Sql Join to retrieve data from more than one table in just one Select statement.

Sql Outer Join
An Sql outer join shows the join, and all the rows that do not qualify.

Sql Self Join
A join that compares values within a single table.

Sql Subquery
A nested Sql Select inside the WHERE clause of another Sql Select statement.

Journey
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